Fluid flow phenomena influence virtually every aspect of the operation of prosthetic blood pumping devices. The efficiency of the device is diminished by viscous and inertial losses in the valves, pumps and conduits. The integrity and health of blood cells - red cells, leukocytes, platelets and others - is impacted by turbulence and shear stress. Responses such as clotting and immune reaction of blood components are influenced by flow conditions. Bloodsurface interactions are affected by wall shear stress and by activation of cells by flow. Embolization of thrombi is related to flow-induced stresses. Furthermore, thrombogenesis itself is correlated to flow stasis and other flow characteristics. Fluid mechanics, through its influence on blood components and thrombus and other mechanisms, may prolong the fight against infection. The reliability of thin diaphragms depends upon folding and stretching patterns determined by flow. Similarly, fluid dynamics play an dominant role in determining cavitation and impact loads and, therefore, the longevity of valves. In the evolution of cardiac assist and replacement devices, we face a point where functionality has been demonstrated through many animal and clinical trials. It is time now to address issues, such as performance, blood-material interactions, infection and control, which will lead to major refinements in designs and procedures. Fundamental to all these challenges is the characterization of fluid flow. Laser Doppler Anemometry (LDA) is the most accurate and convenient technique for measuring flow velocities in most blood pump applications and is the only viable method for many of the geometrically-constrained complex three-dimensional, reversing and unsteady regions of flow in the devices. The University of Utah Institute for Biomedical Engineering, with over $12.4 million in research directly related to development of artificial hearts and ventricular assist devices, recognizes well the importance of understanding and controlling flow in these devices ana is committed to a multi-faceted program of investigation in this area. A conducive environment, organization, facilities and personnel are in place. The LDA system will be the cornerstone of a major research effort.